This invention pertains to time division, multiple access communications systems in which a plurality of stations share a single transmission channel. More particularly, this invention pertains to an adaptive hybrid communications system in which two or more of the contention, reservation and fixed assignment protocols are utilized.
When a communications system includes a plurality of stations transmitting and receiving information on the same channel, the channel must be organized in some fashion to achieve efficient use of the single channel. There are a number of "protocols" that have been used in the past that provide such organization and these protocols are referred to as "multiple access protocols" or "MAPs".
These protocols can be implemented using a central controller, or their implementation can be distributed. With distributed control, each station executes its own protocol routine, which is synchronized and essentially identical to the protocol routines of all other stations on the network. Although the preferred embodiments of the invention described below are directed to MAPs with distributed control, it should be understood that, with minor modifications, implementation of any of these protocols can be performed by a central controller.
There are three basic classes of MAPs: contention, reservation and fixed assignment. Contention protocols are also referred to as "Aloha-type" protocols and fixed assignment as "TDMA-type" (time division multiple access-type) protocols. Consequently, the term "Aloha" will be used throughout the specification and claims to refer to contention protocols, while the term "TDMA" will be used to refer to fixed assignment protocols.
In each of these protocol classes, the channel is usually divided in time into slots and information is transmitted in packets, one packet per time slot. In general, each of these protocols establishes a set of rules that determines which station will transmit its packet of information in a particular time slot. Each of these protocols are described in greater detail below.
MAPs have a variety of applications. For example, among computer communication network, MAPs are used in local area networks (LANs), radio networks, telephone networks, satellite communications networks (SCNs), metropolitan area networks, integrated services digital networks and other well known networks, including hybrid ones. When MAPs are implemented in SCNs, additional complications arise because of the large round trip propagation delay between the transmission of an information packet and the time that information packet is received by the ground stations. For SCNs in which the satellite is in geosynchronous orbit, this propagation delay is typically 0.27 seconds. In contrast, this propagation delay between transmission and reception of an information packet on a LAN may be, for all practical purposes, zero. As a result of this propagation delay, MAPs need special considerations when implemented in an SCN. Accordingly, the preferred embodiments of the invention described below are directed to the implementation of MAPs in an SCN. It should be understood, however, that implementation of any one of the preferred embodiments in another type of network simply requires appropriate adjustment of the propagation delay factors, or, in networks in which the propagation delay is essentially zero, the MAPs described below can be simplified by eliminating the additional steps that compensate for the round trip propagation delay.